1. Field of the Invention
The present invention relates to an optical scanning apparatus, a multi-beam optical scanning apparatus, and an image forming apparatus using the same and, more particularly, to an apparatus suitable for an image forming apparatus such as a laser beam printer or digital copying machine, which can obtain a satisfactory image on a scanned surface (photosensitive surface) by appropriately setting the elements such that the amount of jitter that occurs in the main scanning direction due to a tilt of a deflecting surface of an optical deflector falls within an allowable range.
2. Related Background Art
In an optical scanning apparatus conventionally used for an image forming apparatus such as a laser beam printer or digital copying machine, a light beam emitted from a light source means and optically modulated in accordance with an image signal is periodically deflected by an optical deflector formed by, e.g., a rotary polyhedral mirror (polygon mirror), and focused to a spot on the photosensitive recording medium (photosensitive drum) surface through an imaging optical system having f-xcex8 characteristics, and the surface is optically scanned to record an image.
FIG. 20 is a schematic view showing the main part of a conventional optical scanning apparatus.
Referring to FIG. 20, an almost parallel light beam emitted from a laser unit 201 is incident on a cylindrical lens 202 having a predetermined refracting power only in the sub-scanning direction. Of the almost parallel light beam incident on the cylindrical lens 202, a light component in a main scanning section emerges without any change. In a sub-scanning cross-section, the light beam converges and forms an almost linear image on a deflecting surface 203a of an optical deflector 203 made of a rotary polyhedral mirror. The light beam reflected/deflected by the deflecting surface 203a of the optical deflector 203 is focused onto a photosensitive drum 206 serving as a scanned surface through an imaging optical system (f-xcex8 lens system) 207 having f-xcex8 characteristics, and the optical deflector 203 is rotated in the direction indicated by an arrow A to optically scan the photosensitive drum 206 in the direction indicated by an arrow B (main scanning direction), thereby recording image information.
In recent years, along with size reduction of a printer and the like, an optical scanning apparatus is also required to be compact and have high performance. To meet this requirement, various optical scanning apparatuses have been proposed for which size reduction of the entire apparatus is achieved by bending the optical path using, e.g., a return mirror. Accordingly, sometimes the incident optical system may have to be inclined with respect to a plane perpendicular to the rotational axis of the optical deflector (this system will also be referred to as an oblique incident optical system hereinafter).
In an optical scanning apparatus using such an oblique incident optical system, for example, the light beam incident position on the scanned surface shifts in the main scanning direction due to a tilt of the deflecting surface of the optical deflector, resulting in a shift in drawing position in the main scanning direction, i.e., so-called jitter.
In a recent image forming apparatus having high resolution of 1,200 dpi or more, a jitter amount in the main scanning direction conspicuously appears as degradation in image quality, and the allowable range for the jitter amount in the main scanning direction is narrowing.
A problem posed here is a variation in tilt angle between the plurality of deflecting surfaces.
If the tilt angle varies between the plurality of deflecting surfaces, the drawing position expands/contracts to both sides of the optical axis. For this reason, even when BD detection can be done at the center to minimize the jitter at the center, the drawing position does shift by the above amount.
This will be described in detail. If the tilt angles of adjacent deflecting surfaces are different, the length of a scanning line formed by a light beam reflected/deflected by one deflecting surface is different from that of a scanning line formed by a light beam reflected/deflected by the other deflecting surface. That is, the magnification changes in the main scanning direction.
It is an object of the present invention to provide an optical scanning apparatus using an oblique incident optical system, which can obtain a satisfactory image by appropriately setting the elements such that the amount of jitter that occurs in the main scanning direction due to a tilt of a deflecting surface of an optical deflector falls within an allowable range, and an image forming apparatus using the optical scanning apparatus.
It is another object of the present invention to provide a multi-beam optical scanning apparatus using an oblique incident optical system, which can obtain a satisfactory image by appropriately setting the elements such that the sum of the amount of jitter that occurs in the main scanning direction due to a tilt of a deflecting surface of an optical deflector and the amount of jitter that occurs when a plurality of light sources are used falls within an allowable range, and an image forming apparatus using the multi-beam optical scanning apparatus.
In one aspect of the invention, an optical scanning apparatus comprises an incident optical system for causing a light beam emitted from light source means to be obliquely incident in a sub-scanning cross-section on a deflecting surface of an optical deflector having a plurality of deflecting surfaces, and an imaging optical system for forming, on a scanned surface, an image of the light beam reflected/deflected by the deflecting surface of the optical deflector, wherein letting K (dpi) be a recording density in a main scanning direction on the scanned surface, xcex3 be an absolute value of a maximum difference in a tilt angle between the deflecting surfaces in the sub-scanning cross-section, xcex1 be an incident angle of the light beam incident on the deflecting surface in the sub-scanning cross-section, xcex8 be a scanning angle of an outermost off-axis light beam with respect to an optical axis of the imaging optical system in a main scanning cross-section, f be a focal length of the imaging optical system, and xcfx86 be an angle a scanning central axis makes with an optical axis when an optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section, a condition given by       "LeftBracketingBar"          sin      ⁢              θ        2            xc3x97      sin      ⁢              xe2x80x83            ⁢      γ      ⁢              xe2x80x83            ⁢              (                              cos            ⁢                          xe2x80x83                        ⁢            α            xc3x97            sin            ⁢                          xe2x80x83                        ⁢            γ            xc3x97            cos            ⁢                          xe2x80x83                        ⁢            φ            xc3x97            cos            ⁢                          xe2x80x83                        ⁢                          θ              2                                +                      cos            ⁢                          xe2x80x83                        ⁢            α            xc3x97            sin            ⁢                          xe2x80x83                        ⁢            γ            xc3x97            sin            ⁢                          xe2x80x83                        ⁢            φ            xc3x97            sin            ⁢                          θ              2                                +                      sin            ⁢                          xe2x80x83                        ⁢            α            xc3x97            cos            ⁢                          xe2x80x83                        ⁢            γ                          )              "RightBracketingBar"     less than       6.35          f      xc3x97      K      
is satisfied.
In another aspect of the invention, at least some optical elements of the imaging optical system form some elements of the incident optical system.
In another aspect of the invention, the optical axis is substantially parallel to the scanning central axis when the optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section.
In another aspect of the invention, the further comprises synchronization detection means, arranged near the scanned surface and including a slit, for controlling a timing at a scanning start position on the scanned surface, wherein the slit has an aperture portion inclined in a sub-scanning direction.
In another aspect of the invention, the light beam emitted from the light source means becomes incident on the deflecting surface while having a width larger than a main scanning width of the deflecting surface of the optical deflector.
In another aspect of the invention, in the foregoing apparatus, the recording density in the main scanning direction is not less than 1,200 dpi.
In another aspect of the invention, an image forming apparatus comprises the foregoing optical scanning apparatus and a photosensitive member placed on the scanned surface of the optical scanning apparatus, developing means for developing an electrostatic latent image formed by scanning the photosensitive member with the light beam as a toner image, transfer means for transferring the developed toner image to a paper sheet, and fixing means for fixing the transferred toner image on the paper sheet.
In another aspect of the invention, a multi-beam optical scanning apparatus comprises an incident optical system for causing a plurality of light beams emitted from light source means having a plurality of light-emitting units to be obliquely incident in a sub-scanning cross-section on a deflecting surface of an optical deflector having a plurality of deflecting surfaces, and an imaging optical system for forming, on a photosensitive drum surface, images of the plurality of light beam reflected/deflected by the deflecting surface of the optical deflector, wherein letting K (dpi) be a recording density in a main scanning direction on the scanned surface, xcex3 be an absolute value of a maximum difference in a tilt angle between the deflecting surfaces in the sub-scanning cross-section, xcex1 be an incident angle of the light beam incident on the deflecting surface in the sub-scanning cross-section, xcex8 be a scanning angle of an outermost off-axis light beam with respect to an optical axis of the imaging optical system in a main scanning cross-section, f be a focal length of the imaging optical system, and xcfx86 be an angle a scanning central axis makes with an optical axis when an optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section, R be a radius of the photosensitive drum, and xcex51 and xcex52 be incident angles of light beams which are separated from each other by the largest distance in the plurality of light beams incident on the photosensitive drum surface, a condition given by             "LeftBracketingBar"              2        ⁢        sin        ⁢                  θ          2                xc3x97        sin        ⁢                  xe2x80x83                ⁢        γ        ⁢                  xe2x80x83                ⁢                  (                                    cos              ⁢                              xe2x80x83                            ⁢              α              xc3x97              sin              ⁢                              xe2x80x83                            ⁢              γ              xc3x97              cos              ⁢                              xe2x80x83                            ⁢              φ              xc3x97              cos              ⁢                              xe2x80x83                            ⁢                              θ                2                                      +                          cos              ⁢                              xe2x80x83                            ⁢              α              xc3x97              sin              ⁢                              xe2x80x83                            ⁢              γ              xc3x97              sin              ⁢                              xe2x80x83                            ⁢              φ              xc3x97              sin              ⁢                              θ                2                                      +                          sin              ⁢                              xe2x80x83                            ⁢              α              xc3x97              cos              ⁢                              xe2x80x83                            ⁢              γ                                )                    "RightBracketingBar"        +                  |                              R            ⁢                          (                                                cos                  ⁢                                      xe2x80x83                                    ⁢                  ϵ1                                -                                  cos                  ⁢                                      xe2x80x83                                    ⁢                  ϵ2                                            )                                ⁢          tan          ⁢                      xe2x80x83                    ⁢          θ                |            f         less than       12.7          f      xc3x97      K      
is satisfied.
In another aspect of the invention, in the foregoing apparatus, at least some optical elements of the imaging optical system form some elements of the incident optical system.
In another aspect of the invention, in the foregoing apparatus, the optical axis is substantially parallel to the scanning central axis when the optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section.
In another aspect of the invention, the apparatus further comprises synchronization detection means, arranged near the scanned surface and including a slit, for controlling a timing at a scanning start position on the scanned surface, wherein the slit has an aperture portion inclined in a sub-scanning direction.
In another aspect of the invention, in the foregoing apparatus, the plurality of light beams emitted from the light source means become incident on the deflecting surface while having a width larger than a main scanning width of the deflecting surface of the optical deflector.
In another aspect of the invention, in the foregoing apparatus, the recording density in the main scanning direction is not less than 1,200 dpi.
In another aspect of the invention, an image forming apparatus comprises the foregoing multi-beam optical scanning apparatus, a photosensitive member placed on the scanned surface of the multi-beam optical scanning apparatus, developing means for developing an electrostatic latent image formed by scanning the photosensitive member with the plurality of light beams as a toner image, transfer means for transferring the developed toner image to a paper sheet, and fixing means for fixing the transferred toner image on the paper sheet.
In another aspect of the invention, a multi-beam optical scanning apparatus comprises an incident optical system for causing a plurality of light beams emitted from light source means having a plurality of light-emitting units to be obliquely incident in a sub-scanning cross-section on a deflecting surface of an optical deflector having a plurality of deflecting surfaces, and an imaging optical system for forming, on a photosensitive surface formed from a flat surface, images of the plurality of light beam reflected/deflected by the deflecting surface of the optical deflector, wherein letting K (dpi) be a recording density in a main scanning direction on the photosensitive surface, xcex3 be an absolute value of a maximum difference in a tilt angle between the deflecting surfaces in the sub-scanning cross-section, xcex1 be an incident angle of the light beam incident on the deflecting surface in the sub-scanning cross-section, xcex8 be a scanning angle of an outermost off-axis light beam with respect to an optical axis of the imaging optical system in a main scanning cross-section, f be a focal length of the imaging optical system, and xcfx86 be an angle a scanning central axis makes with an optical axis when an optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section, xcex53 be incident angles of the plurality of light beams incident on the photosensitive surface, and L be a distance on the photosensitive surface between light beams which are separated from each other by the largest distance in the plurality of light beams incident on the photosensitive surface, a condition given by             "LeftBracketingBar"              2        ⁢        sin        ⁢                  θ          2                xc3x97        sin        ⁢                  xe2x80x83                ⁢        γ        ⁢                  xe2x80x83                ⁢                  (                                    cos              ⁢                              xe2x80x83                            ⁢              α              xc3x97              sin              ⁢                              xe2x80x83                            ⁢              γ              xc3x97              cos              ⁢                              xe2x80x83                            ⁢              φ              xc3x97              cos              ⁢                              xe2x80x83                            ⁢                              θ                2                                      +                          cos              ⁢                              xe2x80x83                            ⁢              α              xc3x97              sin              ⁢                              xe2x80x83                            ⁢              γ              xc3x97              sin              ⁢                              xe2x80x83                            ⁢              φ              xc3x97              sin              ⁢                              θ                2                                      +                          sin              ⁢                              xe2x80x83                            ⁢              α              xc3x97              cos              ⁢                              xe2x80x83                            ⁢              γ                                )                    "RightBracketingBar"        +                  |                  L          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢          ϵ3          xc3x97          tan          ⁢                      xe2x80x83                    ⁢          θ                |            f         less than       12.7          f      xc3x97      K      
is satisfied.
In another aspect of the invention, in the foregoing apparatus, at least some optical elements of the imaging optical system form some elements of the incident optical system.
In another aspect of the invention, in the foregoing apparatus, the optical axis is substantially parallel to the scanning central axis when the optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section.
In another aspect of the invention, the apparatus, further comprises synchronization detection means, arranged near the scanned surface and including a slit, for controlling a timing at a scanning start position on the scanned surface, wherein slit has an aperture portion inclined in a sub-scanning direction.
In another aspect of the invention, in the foregoing apparatus, the plurality of light beams emitted from the light source means become incident on the deflecting surface while having a width larger than a main scanning width of the deflecting surface of the optical deflector.
In another aspect of the invention, in the foregoing apparatus, the recording density in the main scanning direction is not less than 1,200 dpi.
In another aspect of the invention, an image forming apparatus comprises the foregoing multi-beam optical scanning apparatus, a photosensitive member placed on the scanned surface of the multi-beam optical scanning apparatus, developing means for developing an electrostatic latent image formed by scanning the photosensitive member with the plurality of light beams as a toner image, transfer means for transferring the developed toner image to a paper sheet, and fixing means for fixing the transferred toner image on the paper sheet.
In another aspect of the invention, an image forming apparatus comprises the foregoing optical scanning apparatus and a printer controller for converting code data received from an external device into an image signal and inputting the image signal to the optical scanning apparatus.
In another aspect of the invention, an image forming apparatus comprises the foregoing multi-beam optical scanning apparatus, and a printer controller for converting code date received from an external device into an image signal and inputting the image signal to the multi-beam optical scanning apparatus.